Designers generally increase performance of integrated circuits by increasing operating frequencies of the circuits and by increasing the number of components, such as transistors, in the circuits. To keep circuit sizes manageable, designers have reduced or scaled down the size of the circuit components so that larger numbers of devices fit within smaller per unit areas. Today it is not uncommon to find advanced computer system chips containing millions, sometimes billions, of transistors. This increased density, however, has created numerous problems. One major problem is heat. Since individual electronic components, such as transistors, each generate minute quantities of heat while operating, increased numbers of such devices in the newer circuits naturally lead to increased quantities of heat.
Compounding the problem of heat is the desire for designers to continually shrink the size of devices containing integrated circuits. For example, designers continually strive to make laptop computers and Personal Digital Assistant (PDA) devices smaller, thinner, and lighter. However, as the cases of these devices shrink, so too do the amounts of available surface areas used for cooling and heat dissipation. Proven circuit designs, based on older and larger circuit platforms, may start having temperature-related problems when they are migrated to different platforms having reduced surface areas for cooling.
Heat and high operating temperatures in integrated circuits cause many problems. First, higher operating temperatures tend to change the operating characteristics of integrated circuits. Second, when integrated circuits are operated at high temperatures for extended periods of time, the long-term reliability of the integrated circuits usually decrease. With these concerns of decreased performance and decreased reliability, coupled with the need of increased component density and smaller packaging, designers increasingly need to monitor the temperature of integrated circuits, including high-performance microprocessors and densely populated application specific integrated circuits (ASICs).
Designers often attempt to manage the temperature of an integrated circuit (IC) by regulating the speed at which it operates. For example, a designer may try to reduce the temperature of a high-performance microprocessor by reducing its operating frequency. Additionally, the designers of an IC may protect the circuit from heat-related damage by either increasing the speed of a cooling fan or possibly shutting down or cutting the circuit off. In order to manage and control the temperature in these different situations, designers must incorporate either external sensors or on-chip sensors that measure temperature.
Using on-chip sensors to measure real-time temperatures helps ensure integrated circuits operate within safe thermal zones. Since many integrated circuits today are becoming increasingly complex, with millions of transistors located within numerous regions of the circuit, designers often desire to know the temperature of several spots in those numerous regions. Until recent times most temperature sensors were based on analog complimentary metal-oxide semiconductor (CMOS) circuits. Temperature measurement with such circuits usually required matched transistors. Additionally, these circuits could not be reliably implemented in numerous spots in integrated circuit designs. Consequently, designers implemented such sensors sparingly, usually limiting the number of sensors to one or two in a given circuit.
Many conventional temperature sensors use analog devices, such as differential amplifiers, that transform or convert the analog temperature signals to digital signals. For example, a large number of existing sensors emit either a current or frequency signal that is proportional to temperature. A frequency output requires a current-to-frequency converter. Unfortunately such converters tend to require large amounts of real estate in an integrated circuit, making such technology a poor choice for designers wanting to make numerous temperature measurements without large circuit footprints.
Designers sometimes use another type of sensor, known as a ring-oscillator sensor, for on-chip temperature measurement. Unfortunately, the ring-oscillator sensor also requires a large amount of integrated circuit substrate space. Plus, the ring-oscillator sensor provides relatively low accuracy. Recently, four-transistor (4-T) decay sensors were proposed. The 4-T sensors are based on a closed-loop process which creates a frequency proportional to temperature. However, the suitable operating temperature range is from +30 to +140 degrees Centigrade.
In light of the growing problem of heat and increased operating temperatures in high-performance integrated circuits, what are needed are alternative methods and apparatuses to measure temperatures of integrated circuits. Alternative temperature sensors need to provide relatively accurate temperature measurements which do not require large footprints.